Lessons from the vegetative shoot apex - Intercellular Communication in Plants - page 162

(Tanaka-Ueguchi et al. , 1998; Hay et al. , 2002). Conversely, increased GA sig-
nalling causes a weak stm mutant to phenocopy a strong stm mutant (Hay et al. ,
2002). Thus the antagonistic interaction between GA and KNOX genes mediates the
balance between indeterminate (meristem) and determinate (organ) cell fates in the
shoot apex (see Fig. 6.3).

6.7 Signals involved in organ formation

Lateral organs arise at regular positions in the periphery of the meristem, a pattern
termed phyllotaxis (from the Greek word for 'leaf arrangement'). The position of
an organ on the stem is often referred to as the node and the space between adjacent
nodes - the internode. Nodes may either contain a single organ, a pair of organs or a
group of organs. In some cases, each adjacent organ arises directly (180 ◦ ) opposite to
the earlier formed organ, called distichous phyllotaxis .Anexample of this pattern is
the arrangement of leaves along the maize stem. More common is spiral phyllotaxis ,
where adjacent nodes are frequently offset by 137 ◦ .For example Arabidopsis

leaves and flowers are arranged spirally around the stem in either a left- or right-
handed pattern. When two organs arise directly opposite each other at a single
node, the arrangement is called opposite phyllotaxis .Inthe majority of cases nodes
are spirally arranged, although occasionally they may be separated by 90 ◦ , termed
decussate (for example see Fig. 6.1A). The term whorled is often used to describe
more than two organs arising at a node, as seen in the flowers of many species.

∼

6.7.1 Models of phyllotaxis
How phyllotaxis is established and maintained has puzzled generations of biologists
and mathematicians (Steeves & Sussex, 1989). Early observations of phyllotaxis
recognised that organs tend to initiate in the greatest space available and at a point
farthest from established primordia and the central zone of the meristem - termed
Hofmeister rule (Snow & Snow, 1962). Indeed, a spontaneous or induced change in
the position of a single organ is sufficient to change the position of all subsequent
organs and may even cause a shift in the handedness of the spiral. This shows
that existing primordia somehow influence the position at which lateral organs will
subsequently form. Classic surgical and pharmacological experiments suggest that
organs produce some kind of chemical or physical signal that determines the position
of subsequent organs (extensively reviewed in Steeves & Sussex, 1989; Lyndon,
1990). Based on these observations an inhibitory field theory was proposed, in
which existing primordia and the central zone of the meristem produce a diffusible
signal that inhibits the formation of new organs. It follows that new primordia will
arise only at a point farthest from the source of this signal in a region of least
inhibition (Schoute, 1913; Wardlaw, 1949). An alternative model proposes that the
signal promotes organ formation and that it is depleted from tissue surrounding
developing primordia. Accordingly, organ initiation will occur only when the levels